“It’s beautiful, it changes over time, it’s warm, it’s healthy.” Anne Cecilie Haug, senior architect at international practice Snøhetta, is extolling the virtues of building in engineered timber. The material’s elegance and environmental virtues are attracting more and more of her peers to work with it.
A common form of engineered wood is cross-laminated timber (CLT) — panels up to 40cm thick made from layers known as lamellas — usually three to five per panel — of renewable softwood glued with the grain of each layer at a right angle to the one below and above. Another is glulam, made by bonding individual laminations of solid timber sections to make beams and other structural members. In these layered forms, a kind of elephantine plywood, timber is far stronger than in its natural state.
One of Haug’s latest projects in engineered timber was a house for construction engineer Tor Helge Dokka. Its black timber-faced upper storey — made of arched glulam ribs on a CLT platform — juts out elegantly on timber struts from a hillside overlooking the town of Kongsberg, south-west of Oslo.
Dokka commissioned the structure, all made from Norwegian and Swedish spruce, and assembled it with his family in three days. “I wanted a low-carbon building with low energy use,” he says of his priorities for his new home. He estimates the timber construction has 70 per cent less embodied carbon emissions than an equivalent 190 sq m house in concrete and brick.
Dokka says one of the pleasures of the house is how the structure explains itself. “You can read the building, how it is engineered,” he says. This simplicity of construction — CLT wall panels bear the weight of the floors above — is a recurring theme among engineered timber advocates.
“It’s the honesty of it,” says architect Laura Dewe Mathews. “You are not hiding things; it’s just blocks you are putting in place.” In 2013, Dewe Mathews bought a former car repair shop behind an end-of-terrace Victorian house in Hackney, east London, that had once been home to a box maker to the jewellery and perfume trade. Into the workshop’s dilapidated walls, she dropped her own 80 sq m CLT box to make her family home.
The 31 timber panels, 9cm and 12cm thick, were made of spruce to her specifications and shipped from Austria, craned into place in a single day and then permanent fixings applied over the following week. Dewe Mathews, who says the CLT build added about 3 per cent to the project cost over conventional block construction, had made a shoebox-sized architect’s model of the house out of cardboard. She says watching the panels being placed on top of each other was “like my model had turned up on the site”.
Since insulation and weatherproofing is applied to the outside of the panels, the interior surface, which can specified to a finer finish, can be left exposed. Like many who choose bare timber, Dewe Mathews treated the walls with a clear compound to resist stains and prevent it darkening too much over the years, preserving the pinkish blond warmth of the spruce.
For anyone who feels a bare timber interior would be too reminiscent of a ski chalet or a sauna, there is always the option of painting — as all the houses covered here have done with selected walls, or even covering in plasterboard. But to conceal the natural finishes would be to forgo some benefits.
“One of the reasons we find it so relaxing to be around [bare] timber is the lack of order to it,” says Andrew Waugh, a founding partner of Waugh Thistleton. “It is so particularly random that our minds stop looking for patterns.” But Dewe Mathews says she still enjoys pattern seeking, mapping the “constellations” of knot holes on the pitched roof of her bedroom.
Waugh is a flag carrier for engineered timber in high-density housing. His practice — named 2023 Architect of the Year by Architectural Review — designed what was then the world’s largest CLT development in 2017 in Dalston, east London: a 10-storey, 121-apartment complex. At that point he estimated there were 500 completed CLT buildings in the UK.
“We think there are about 800 now, but the majority since 2017 are single family houses and small or temporary buildings,” he says. Multistorey projects stopped that year, after the Grenfell Tower fire in London when combustible cladding retrofitted to the concrete and steel high-rise contributed to the deaths of 72 people. Though the solidity of the engineered timber and a protective charring of the outer surface makes it relatively slow to burn, multistorey CLT structures were caught in a blanket ban on combustible facades on higher residences.
Waugh Thistleton has developed an open-source building model combining engineered timber frames with non-combustible facades that has now been approved by the National Housebuilding Council for projects up to six storeys. Waugh says the practice is in discussion with developers and local authorities about multistorey development again. He is hopeful that CLT will contribute to the solutions to both the climate and housing crises.
“We are looking to present this optimistic view that these are healthy, beautiful buildings that are built quickly from replenishable materials,” he says. He points to studies that associate timber interiors with health benefits for occupants including lower blood pressure and increased sense of wellbeing.
As well as the environmental benefits in carbon saved, engineered timber offers opportunities for creative waste reduction. Anne Cecilie Haug made a dining table from the CLT cut out for one of the window apertures at a tourist lodge she designed. For a project to enlarge a 1960s three-bedroom house in Leyton, east London, in 2022, architects Unknown Works repurposed all the offcuts they could to make built-in benches, tables, a pivoting front door and even chunky dimmer switches for the lights. They added 29 sq m — almost doubling the internal space of the ground floor — by adding banana-yellow painted CLT boxes on the front and back.
The brief was to create extra space for the young family of clients who wanted something “sustainably focused and playful”, according to the practice’s director and co-founder Ben Hayes, who says the project gave him and his colleagues a “real taste” for CLT. They are building a demountable pavilion all in cross-laminated panels, like a giant hollowed-out pine cone, that they hope will form the stage at music festivals next year.
Twelve years after moving into the CLT house she designed, Laura Dewe Mathews shares Hayes’s enthusiasm. “I would use it straight away on other projects,” she says. After some weeks away recently, she says, coming home to the timber interior was a joy: “I was quite affected by it, seeing it afresh . . . it gives you a sense of cosiness. To be enveloped by wood, it’s the closest we get to a womblike environment.”
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